Do you know what design BRE digest 365 means? and why it is used?

Soakaways have been the traditional way to dispose of stormwater from buildings and paved areas remote from a public sewer or watercourse. In recent years, soakaways have been used within urban, fully sewered areas to limit the impact on a discharge of new upstream building works and to avoid costs of sewer upgrading outside a development. Soakaways are seen increasingly as a more widely applicable option alongside other means of stormwater control and disposal. Soakaways must store the immediate stormwater run-off and allow for its efficient infiltration to the adjacent soil.

They must discharge their stored water sufficiently quickly to provide the necessary capacity to receive run-off from a subsequent storm. The time taken for discharge depends upon the soakaway shape and size, and the surrounding soil’s infiltration characteristics. They can be constructed in many different forms and from a range of materials. This Digest describes design and construction procedures, explains how to calculate rainfall design values and soil infiltration rates, and gives design examples Soil Infiltration Rate Excavate a soakage trial pit to the same depth as anticipated in the full-size
soakaway; for run-off from 100m2 this will be 1 to 1.5 m below
the invert level of the drain discharging to the soakaway. Overall
depths of excavation will be typically 1.5 to 2.5 m for permeable areas up to 100m2 draining to the soakaway. The trial pit should be 0.3 to 1 m wide and 1 to 3m long. It should have vertical sides trimmed square and, if necessary for stability, should be filled with granular material. When granular fill is used, a full-height,
perforated, vertical observation tube should be positioned in the pit so that water levels can be monitored with a dip tape. It should be possible to construct a suitably dimensioned pit with a backhoe loader or mini excavator. Narrow, short pits use less water for the soakage tests but may be more difficult to trim and clean prior to testing. Measure the pit carefully before trials.

The inflow should be rapid so that the pit can be filled to its maximum effective depth in a short time, ie to the design invert level of the drain to the soakaway. Take care that the inflow does not cause the walls of the pit to collapse. Fill the pit and allow it to drain three times to near empty; each time record the water level and time from filling, at intervals sufficiently close to clearly define water level versus time. The three fillings should be on the same or consecutive days. Calculate the soil infiltration rate from
the time is taken for the water level to fall from 75% to 25% effective
storage depth in the pit, using the lowest f value.